CA2075954A1

CA2075954A1 - Optical fiber having an aspherical lens thereon

Info

Publication number: CA2075954A1
Application number: CA002075954A
Authority: CA
Inventors: David R. Thompson
Original assignee: BT&D Technologies Ltd
Current assignee: BT&D Technologies Ltd
Priority date: 1990-01-31
Filing date: 1991-01-30
Publication date: 1991-08-01
Also published as: US5037174A; AU7320391A; EP0513202A4; AU640995B2; DE69126202D1; EP0513202B1; KR920704163A; JPH081488B2; EP0513202A1; JPH04507152A; WO1991011739A1; DE69126202T2

Abstract

An optical fiber (F) having an axis therethrough and having a tip thereon comprising a first tapered region and a second adjacent tapered region (52A, 56A). The second tapered region (56A) terminates in an aspherical lens (60). Each of the tapered regions (52A, 56A) has a surface thereon. The surface of the first tapered region (52A) defines an angle with respect to the axis of the fiber (F) that lies in the range from about ten (10) to about thirty (30) degrees, while the surface of the second tapered region (56A) defines an angle with respect to the axis of the fiber that lies in the range from about thirty-five (35) to about sixty (60) degrees. The second tapered region (56A) is produced as a result of a jerking action imposed during the drawing of the fiber (F).

Description

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`~ TITLE
OPTICAL FlBER HAVING AN ASPHERICAL LENS THEREON
BACKGROUND OF THE INVENllON

Field Of The Invention This inYention relates to an optical fiber having an aspherical lens at the tip thereof and to l O a method of making the same.

Description Of The Prior Art An op~ical communication : system inc~udes a source of light energy and an associated ~- receiver connected over an optical fiber waveguide. The - 15 measure of the power coupled from the source into the fiber or from the fiber to the receiver is termed coupling efficiency.

~: The typical optical fiber has a core approximately nine , ~ (9) micrometers in diameter, the core being formed of a `- 20 material that exhibits a first predetermined index of refraction.
~` The core is surrounded by an outer layer of a cladding material ~: that exhibits a second predetermined index of refraction. The . overall outer diameter of the typical optical fiber is on the ~; order of one hundred twenty five ( 1 25) micrometers.
The optical fiber usually has a lensed end at its tip. The lensed end is typically spherical, although it is known that the lensed end may be aspherical in shape.
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Although a spherical lens is easy ~o produce and is - generally sufficient to the meet the needs of the system in . which it is placed, a spherical lens is subject to spherical aberration. Such spherical aberration lowers coupling efficiency and thus renders such a fiber less preferred for low ~1 3 5 loss, high gain uses, such as laser optical ampli~lers.

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",` . .' ' , ' "" ' ' . , ` " WO 91/11739 PCr/US91/00473 .` 2 Il has been recognized thal an aspherical lens reduces sphcrical aberration and improves coupling efficiency for any sized lens. However, an aspherical lens does not appear to be widely used on optical fiber waveguides, perhaps because of a : 5 perceived difficulty in manufacturing the same.

United States Patent 4,565,558 (Keil et al.) and United -States Patent 4,~89,897 (Mathyssek et al.) both relate to the formation of a spherical or asl,herical lensed end on an optical -l O fiber. The apparatus disclosed in these patents u~ilizes two clamps, at least one of which moves relalively to the other while an electric arc heats a portion of the fiber between the : clamps. As a constriction appears as the result of constant tension and heat, the tension is dropped and a further constriction occurs leading to a separation which solidifies .. when the heat is cut off to form a lens on a tapered fiber.
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Blaudau and Rossberg, Journal of Lightwave Technology, Vol. LT-3, No. 3, April l985 teach making an aspherical lens by .. 20 first forming a bulbous spherical lens on a fiber and then ; welding a cylinder of pure quartz at the center of the bulb.
:~; Upon remelting the pure quartz flows out to form an aspherical surface .
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2 5 United States Patents 4,243 ,349 and 4,370,021 (both to ~; Khoe et al.) teach flattening the end of an optical fiber to produce a semi-elipsoidal lens.

, ` SUMMARY OF THE TNVENTION
-~ 30 The present invention relates io an optical fiber waveguide with a ~ip having two tapered regions ~hereon. The first tapered regio~ is disposed adjacent to ~he full diameter of the fiber and is characterized as being shallow in slope with . 3 5 respect to the axis of the fiber. Defined more precisely an '~' .. -. . :
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WO 91/1 1739 PCl'/US91/00473 207a9~4 extension of a line Iying on the surf~ce of Ihe first tapered region intersects with the ~xis of ~he fiber at an angle in the range from ten (10) to thirty (30) degrees. Preferably, ~he angle is about eighteen ( 18) degrees.
A second, more steeply tapered region is disposed immediately adjacent to the first tapered region. The second region is characterized in that an extension of a line Iying on the surface thereof intersects with the axis of the fiber at an 10 angle in the range from thirty-five (35) to sixty (60) degrees.
Preferably, the angle is about of forty five (45) degrees. The second, more steeply tapered region terminates in an ~" aspherical lens. The lens is preferably substantially hyperbolic ' in section.
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The present invention also relates to a method for making an optical fiber waveguide having a tip of the above described structure. The method includes the steps of positively clamping a length of an optical fiber at first and 2 0 second spaced clamping points and directing an energy arc at a ; predetermined separation point on the fiber intermediate ~he first and second clamping points. The arc has a temperature sufficient to soften the fiber above its transition temperature.
There is thus defined with respect to the separation point a 25 first and a second portion on the fiber. At least one of the clamps is relatively moved with respect to the other clamp at a first predetermined separation acceleration in the presence of the energy arc thereby to define a first tapered region on at least one portion of the fiber. The separation acceleration is 30 stepwise increased to jerk apart the fiber and separate the first and second portions at the separation point. This action forms a nipple-like extension on at least the portion of the fiber having the first tapered region. The portion of the fiber having the nipple-lilce extension thereon is cooled below the Iransilion 35 temperalure to solidify the extension. Thereafter the solidified .~
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. , , WO91/11739 PCI'/US91/00473 9~ 4 nipple-like extension is introduced into the arc. The nipple-like extension is healed more quickly than lhe remainder of ~he tip and, owing to surface tension effects, the nipple-like extension withdraws and contracts to form into lhe second tapered region having an aspherical lensed end. The more pronounced slope of the second tapered region is formed as a result of the jerked separation of the fiber into the first and second portions.
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BRIEF DESCRIPTION OF THE DRAWlNGS

The inven~ion will be more fully understood from the following detailed description thereof, taken in connection with ; the accompanying drawings, which form a part of this application, ar~d in which:
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Figure 1 is a highly stylized pictorial representation of an apparatus for implementing the method of forming an optical ;, fiber having an aspherical lens in accordance with the present invention;
~" .,; ., Figure 2 is a timing diagram of the state of various ; parameters of the formation process and illustrations of the - ~ profile of an optical fiber being formed at various -- 25 predetermined times during the practice of the process in .i accordance with the present invention;
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--- Figures 3A and 3B are selected enlarged views of the profile of the tip of an optical fiber shown in Figure 2; and i 30 ;; Figure 4 is an enlarged profile view of the tip of an optical fiber having an aspherical iens thereon formed in ~` accordance with the present inven~ion.
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3 5 DETAlLED DESCRIPTION OF THE lNVENTlON
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: 5 ~ -Throughout the following detailed description similar reference numerals refer to similar elements in all figures of - the drawings.

Figure I is a is a highly stylized pictorial representation of an appar~tus generally indicated by the reference charac~er . 10 for implementing the method of forming an optical fiber having an aspherical lens in accordance with the present - 10 invention. The apparatus 10 includes a first and a second fiber clamp 14A, 14B e~ch of which is mounted on a respective stage 16A, 16B. As will be discussed each clamp 14A, 14B is operative to positively clamp a predetermined length of an optical fiber F at respective first and second clamp points ~OA.
lS 20B. The clamp points 20A, 20B are spaced apart to define a draw zone generally indicated by the reference character 2~.

The clamps 14A, 14B are preferably implemen~ed using a ~: grooved platform 17A, 17B in which the fiber is laid. The fiber 2 0 is positively held in place so as to preclude slippage by an overcenter clamp. Each of the stages 1 6A, 1 6B may be implemented using a linear stage device such as that available from Klinger Scientific of Garden City, ~ew York as the Klinger model UT 100.25 linear stage.

- - Each stage 16A, 16B has a respective drive motor 26A, 26B suitably secured thereto. The drive motors 26A, 26B are operalive to translate each stage 1 6A, I 6B along a respective path of travel, indicated in Figures I and 2 by the respective 30 reference character 28A, 28B. Each motor 26A, 26B is controlled by a suit~ble motor controller, collectively indicated - by the reference character 30. It should be noted that although in the preferred embodiment being described both of the stages may be translated, it lies withirl the contemplation of 35 the invention to fix (or, at least not move) a given one of the ;.
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W O 91/11739 PC~r/US91/00473 9~ 6 stages~ or to utilize any other expeditious arrangement so long as one of the stages is relatively movable with respect to the : other at some predetermined acceleralion.

Suitable for use as the drive motor 26A, 26B is the ; stepper motor available from the Compumotor Division of Parker Hannifin Corp, Petaluma, California as Compumotor A57-83. The motor controllers 30 may be implemented using the controllers available from the same source as Compumotor l O A57-83 controllers.
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~` Disposed in the draw zone 22 is a pair of arcing f electrodes 34A, 34B. The electrodes 34A, 34B are each ; connected to a suitable arc transformer and controller, l 5 collectively indicated in Figure I by the reference character 38.
Suitable for use for this purpose is the device available from Action Instruments Inc., San Diego California. Each arc transformer and controller 38A, 38B includes a step-up transformer, an AP3231 Phase Angle Controller and an AP3010 :.~ 20 Inductive Load Driver. The transformer steps up a 240 volt 50 Hz power input to 5000 volts while the phase angle controller ; ~ and the driver function as a sine wave chopper on the power output from the transformer.

2 5 Overall system control is effected using a suitable `;- programmable control device 40, such as an IBM lndustrial AT
(operating on DO 3.1) which is obtainable from International ; Business Machines Corp. Manufacturing Systems Products in . ~ Boca Raton, Florida. The computer 40 is equipped with two ;` 30 Compumotor PC 21 cards and a Data Translation twelve-bit D/A
.`, card. The latter is obtained from Data Translation, Marlboro, Massachusetts.
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The apparatus 10 also includes a visual monitoring 3 5 system generally indicated by the reference character 42 for ., ~'.`

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WO91/11739 PCl`/US91/00473 2o~ra viewing the region of the draw zone 22 in the vicinity of the separation point S on the fiber F. The visual monitoring system 42 includes a suitable camera 44 and associated optics 46 (to : provide the desired level of magnification) whereby a real time5 picture of the lens formation may be viewed by the operator at a monitor 48. The vicinity of the separation point S in the lens forming area of the draw zone 22 should be backlit with ." colluminated white light.

. I0 To form an aspherical lens on the tip of an optical fiber F, the jacket and buffer coating of a predetermined length of the fiber F is stripped and cleaned. The fiber F is clamped by the clamps 14A, 14B at the respective clamp points 20A, 20B so that a length of the cleaned and stripped fiber traverses the ` 15 draw zone 22. The remaining length of ~he fiber F still covered r' with the jacket and buffer coating is held out of the way, as represented by the coiled length C shown in Figure 1. In the Figures that follow herein the upper member is to be understood as part of this remaining coiled length of fiber. The 20 lower member in the Figures defines a stub length of fiber that is usually discarded.
.: -A predetermined point S on lhe fiber F in the draw zone 22 lies between the electrodes 34A, 34B. The point S, as will 25 be developed, defines a separation point at which a first and a second portion of a separated fiber will be defined.
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;` The steps of the method in accordance with the present invention may be understood from Figure 2. This Figure is a 30 timing diagram of the state of various parameters of the formation process and illustrations of the profile of the tip an optical fiber being formed at various predetermined times during the practice of the process in accordance with the presenl invention. In Figure 2 lhe relative acceleration, 35 velocity, and displacement of the clamps 14A, 14B and the 7"

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are indicated as a function of time. Moreover, the Figure 2 illustrates the physical changes undergone by the leng~h of the fiber F about the point S within the draw zone 22.
At the initia~ion of the process an arc is produced across `~ the electrodes 34A, 34B under the control of the computer 40.
:.-. The arc interacts with the fiber in the draw zone at the separation point S. The arc energy is controlled throughout the process so that the energy of the arc: ranges from an initial arc energy value to a final arc energy value during drawing of the fiber; thereafter jumps to a jerking arc energy value; and is ~; later reduced to exhibit a predetermined lensing arc energy, all .; as will be explained. A predetermined "bit count" (a measure : 15 used in the Table that follows herein) in the range from 0 to 4095 from the computer 40 corresponds to a 0 to 10 volt output to the controller 38 and produces a corresponding 0 to 50 mA current to the electrodes 34A, 34B. The current to the ; electrodes governs the arc energy level.

At the start of the process the clamps 1 4A, 1 4B are relatively moved away from each other at a predetermined -.` constant taper acceleration. This drawing action defines a long, relatively shallow slope taper 52A, 52B on each portion of the fiber F above and below the separation point S. As will be explained, the slope of the taper is defined wilh respect to the ; axis A of the fiber F (Figure 4). As the clamps start to move the arc energy is reduced by the controller 40 from its initial :~ value toward the final value at a constant rate as the fiber is ~`~ 30 drawn thinner. During this phase the arc energy is selected to apply the minimum heat necessary to raise the fiber above its . transition temperature ~o permit the drawing to occur. Too intense of an arc energy will result in the fiber being melted through, while too little heat will no allow the fiber to draw . 35 properly.
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., , "~', . ., -- : , WO91/lt739 207 9~ ~ PCI-/US91/00473 When lhe diameter dimension D (Figures 2 and 3A) of the fiber at thc separation point S reduces to a predctermined cross-sectional dimcnsion (found to be approximately twenty 5 micrometers for a one-hundred-twenty-five micrometer fiber) the acccleration of the c]amps 14A, 14B is stepwise increased.
The stepwise increase is an abrupt change in acceleration from the initial taper acceleration to the jerk acceleration value occurs over a short lime span, on the order of milli-seconds.
10 The relative displacement of the clamps at the imposition of the jer~;ing action is termed the taper distance and is shown graphically in Figure 2.

The stepwise increase in acceleration resul~s in a jerking 15 action that is imposed on the fiber and causes the first portion 54A and the second portion 54B thereof to separate at the separation point S, as seen in Figure 2. The jerking aclion sharply changes the slope of the taper, as is best seen at reference characters 56A, 56B in the Figures 2 and 3A.
2 0 Moreover, the jerking separation of the fiber into the first and second portion pulls out a nipple-like extension 58A, 58B of material as the fiber separates into two parts.
., . Once the fiber is separated into the first and second 2 5 portions 54A, 54B, the arc is extinguished and the working ~: length of the fiber (i.e.. the upper coiled portion C of the fiber shown in the Figure 2) is moved baclc toward the plane of the arc, as indicated in Figure 2 by the arrow 60. During this time - the end of the portion ~4A of the fiber F and the nipple~ e 3 0 extension 58A lhereon are allowed to drop below the transition : temperature. The duration of the cooling period, as well as the durations of all the other time periods, are shown on the graphs of Figure 2.
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WO 91~1 17~ 9~ 0 Pcr/US91~00473 The arc is reigniled and the operator, guided by the moni~or 48, applies one-half sccond bursts of arc (at a lènsing `
arc energy value) to the thc nipple-like extension 58A on the firsl por~ion 54A of lhe fiber F. The applicalion of the lensing arc softens the material in the nipple-like extension 58A to a greater extent than to the tapered portions 52A, 56A of lhe first portion ~4A of the fiber F. The nipple-like extension 58A
is thus raised above its transilion temperalure, and surface tension effects cause the extension 58A to withdraw upwardly 10 (as viewed in the Figure 2, 3A and 3B). The conlraction and coalescence of the nipple-like extension 58A to form a second tapered region 56'A having a lens 60 (Figures 3B, 4) at the end - thereof is believed best seen by comparison of Figures 3A and - 3B.

It should be understood that the applicalion of the lensing arc energy may be carried out under program control, ; and that the application of a single burst of arc (for the appropriale duration and at the appropriate energy) to form 20 the seond tapered region 56'A and the lens 60 lies within the ~;i contemplation of the invention.

The resultan~ lensed fiber thus formed is shown in the -- enlarged view in Figure 4. The optical fiber produced as a' ~` 2 5 resull of the present invention has a tip having two tapered regions thereon 52A and 56'A thereon. The first tapered region 52A is disposed adjacenl to the full diameter of the fiber (not visible at the scale of Figure 4) and is characterized as : being shallow in slope wilh respecl to the axis A of the fiber.
3 0 The surface of the firsl tapered region 52A defines a first ~' predetermined angle 64 with respect ~o the axis A of the fiber r., ' F. As more precisely seen in Figure 4 an extension of a line Iying on the surface of the first tapered region 52A intersects with the axis A of the fiber at an angle 64 in the range from ' .

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Wo 91tll739 1 1 2 0 7 ~ 9 5 4 pcr/us91/oo473 ten ( 10) to thirty (30) degrees. Preferably, the angle 64 is about eighleen (18) dcgrees.

The second tapered region 56'A of the tip of the fiber F
is disposed immediately adjacent to the first tapered region 52A and lhe surface of this region is more steeply tapered with respect to the axis A of the fiber F. As noted above, this second region 56'A is produced as a result of the jerking action which separates the fiber into the first and second portions. The second tapered region 56'A is characterized in that an extension of a line Iying on the surface thereof intersects wi~h the axis A of the fiber at an angle 68 that lies within the range of thirty-five (35) to sixty (60) degrees. Preferably the angle is about forty five (45) degrees.
The second, more steeply tapered region 56'A terminates in an aspherical lens 60. The lens 60 is preferably substantially hyperbolic in section. In three dimensions, the lens 60 is preferably hyperboloidal. lt should be understood that the terms "substantially hyperbolic in section" and "hyperbo~oidal" are meant to encompass a lens shape that deviates from being hyperbolic in section and hyperboloidal in shape and tend toward being parabolic in section and paraboloidal in three dimensional shape.

. It will be recognized that the interaction of the parameters is such that a certain amount of experimentation is re~uired to select the conditions that yield a desired configuration. The following tabulation shows a number of ; 3 0 examples of such variation and ~he results achieYed in terms ofthe average radius of the aspherical lens. "Average radius"
means the radius of a circle fi~ by a least squares analysis to the profile of the tip of the fiber (i.e., the profile shown in Figure 4) centered on a point X disposed approximately fifteen (15) micrometers alon~ the axis A from the tip of the lens 60.
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WO 9~/11739 ' PCI`/US91/00473 TABLE

AVERAGERADIUS 8 l 0 l 4 : 5 (micrometers) TAPER DISTANCE 12 ~ 80 90 (micrometers) INlTlAL ARC 3000 2800 2800 (bit count) : FINAL ARC 2223 227~ 2350 (bit count) . TAPER ACCEL. 4.6 10.0 10.0 ~
:.; (micrometers/sec2) - ' -: 20 (bit count) ;.
: JERKACCEL. 3000 8000 8000 ~'~.. , (micrometers/sec2) . .
,,. . ~jj Aspherical lenses made according to the present ` invention exhibit superior properties (divergence angle and -. coupling efficiency, without significantly greater sensitivity to :-- lateral misalignment) whcn compared with sphcrical lenses of . the same average radius. While the greatest utility of this -.` 30 invention is believed to lie in lensing a monomode fiber, the invention can also be applied to a multimode ~lber if desired.
Those skilled in the art, having the benefit of the teachings of :: the present invention, may impart numerous modifications , .
t:.''' thereto. It should be understood that such modifications lie ; :

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~, . . : -Wo 91/11739 2 o 7 ~ 9 ~ ~ PCl`/US91/00473 within the contemplation of the present invcnlion, as defined by the appended claims.

WHAT IS CLAlMED IS:

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Claims

1. An optical fiber having an axis and having a tip thereon, the lip comprising a first tapered region and a second adjacent tapered region, the second tapered region terminating in an aspherical lens, each of the tapered regions having a surface thereon, the surface of the first tapered region defining an angle with respect to the axis of the fiber that lies in the range from about ten (10) to about thirty (30) degrees, the surface of the second tapered region defining an angle with respect to the axis of the fiber that lies in the range from about thirty-five (35) to about sixty (60).

2. The fiber of claim 1 wherein the angle defined by the first tapered region is about eighteen (18) degrees.

3. The fiber of claim 2 wherein the angle defined by the second tapered region is about forty-five (45) degrees.

4. The fiber of claim 1 wherein the angle defined by the second tapered region is about forty-five (45) degrees.

5. The fiber of claim 4 wherein the aspherical lens is substantially hyperbolic in section.

6. The fiber of claim 3 wherein the aspherical lens is substantially hyperbolic in section.

7. The fiber of claim 2 wherein the aspherical lens is substantially hyperbolic in section.

8. The fiber of claim 1 wherein the aspherical lens is substantially hyperbolic in section.

9. A method of forming a tip on an optical fiber comprising the steps of :

(a) positively clamping a length of an optical fiber at first and second spaced clamping points, (b) directing an energy arc at a predetermined separation point on the fiber intermediate the first and second clamping points to define with respect to the separation point a first and a second portion on the fiber, (c) relatively moving at least one of the clamps with respect to the other at a first predetermined separation acceleration in the presence of the energy arc thereby to define first tapered region on at least one portion of the fiber, (d) stepwise increasing the separation acceleration to jerk apart the fiber and separate the first and second portions and to form a nipple-like extension on at least the portion of the fiber having the first tapered region, (e) cooling the portion having the nipple-like extension thereon below its transition temperature, and (f) thereafter introducing the nipple-like extension into the arc to form the nipple-like extension into a second tapered region having an aspherical lensed end.

10. The product produced by the process of claim 9.